Relationships Between Invertebrate Phyla Based on Functional-Mechanical Analysis of the Hydrostatic Skeleton

Abstract

SYNOPSIS. The phylogeny of the major groups of deuterostome coelomates—the chordates,hemichordates and echinoderms—is discussed based on a mechanical-functional analysis of the hydrostatic skeleton and associated structures. The basic approach is to first establish transformation series of individual features and of functional complexes of features and secondto determine their “Lesrichtung” by showing the direction of increased economy (i.e., better adaptation) with respect to environmental factors. It is argued that a metameric coelom is primitive with respect to an oligomeric one and that the ancestral form of the deuterostome coelomates is a metameric, coelomate worm-like animal with a complex set of circular, transverse andlongitudinal body muscles. The coelom plus the complex body musculature formed the hydrostaticskeleton. The sequence of structural modifications leading to chordates is: (a) appearance of the notochord; (b) specialization of the dorsal longitudinal muscles with a reduction and disappearance of the transverse and circular muscles; (c) simultaneous appearance of the dorsal hollow nerve cord; (d) development of a postanal tail; and (e) appearance and specialization of the branchial basket with gill slits as a filter feeding apparatus. The primitive chordate would be most similar to the lancelet (Acrania). Tunicates are advanced chordates specialized forsessile life and lost most chordate features in the adult, but retained them in the larvae as adaptations for active dispersal. Enteropneusts (acorn worms) are another advanced group specialized for burrowing in fine sediments and that evolved the anterior proboscis as a peristaltic burrowing organ. The notochord was lost as was the dorsal nerve cord and segmented conditionof the coelom. A collar originated as a means to prevent discharged water from re-entering themouth. Pterobranchs arose from enteropneustlike forms; their major structural changes are reduction of the branchial basket and modification of the collar into tentacles which are associated with life in a closed tube. Finally, echinoderms arose from a pterobranch-like ancestor by specializing for sessile life and feeding with tentacles and by final loss of the branchial basket. Groups such as the tunicates, hemichordates and echinoderms could be eliminated as ancestral forms within the deuterostome coelomates because the evolution of acraniates and vertebrates from each of these groups would involve the appearance of gill slits before the notochord and/or the evolution of a metameric coelom from an oligomeric one, both of which are exceedingly improbable. Central to the methods used to establish the transformation series of features and their direction of evolutionary change (Lesrichtung) are functional (mechanical) analysis and adaptive interpretation of features; hence, functional-adaptive analyses are an integral and essential part of the methodology of phylogenetic investigation.